If you're referring to the 'Western' Allies; supply lines. Vladivostok is only a good staging ground in theory; how to get supplies for the build up there? You'd have to first cross the dominated sea of Japan to attack the Japanese home Islands; a threat that would undeniably bring down the complete available force of the Japanese in reaction. A reaction that would be carried along interior lines, and be far more rapid than anything the Allies or the USSR could do.

Then we have more grounded issues beyond the mind-numbingly exposed supply line; who would invade? The British and Commonwealth were fighting in the Subcontinent and in New Guinea, tying down what forces they had in the Pacific and Oriental theaters. The Americans were fighting a two front War, and there's a reason why their strategy was based on slowly but surely 'hopping' into range of Japan; secure supply lines and a friendly sea.

Finally, the USSR was in a fight for its very existence in its European heartland, and was moving Eastern units West to form reserves against the onslaught. They had neither time, infrastructure (their Navy being rather incapable of effective, massive amphibious landings), or manpower to be capable of said operation. Further, the Japanese had shown no interest in invading the mainland USSR after the launch of Barbarossa and it would have been in Russia's worse interests to encourage Japanese intervention or aggressive reaction (if, for example, the build up was detected) at a time when all their efforts and nerves were strained on fighting the Wehrmacht. The Russian VVS (Air Force) would be nearly non existent in presence, and the idea of getting several Carrier Groups into striking distance would be near impossible as well; to say little of the logistical farce it would become to try and supply a US Carrier Group with Russian depots. There's a reason the US leased equipment to the USSR, and not vice-versa, is what I'm getting at; it would've been an impossible strain.

TLDR; It would've been a stretch of Strategic fancy more belonging to the Napoleonic era, and a complete and ineffective diffusion of forces.

Keep in mind that the Soviets weren't at war with Japan until August 8, 1945. They had no reason to want to allow an invasion through their territory of someone they weren't at war with, and no inclination to deal with Japan until the (to their mind) far more serious threat of Germany was dealt with.

That's in addition to the excellent points made by the wonderfully-named /u/BritainOpPlsNerf (I love that name so much).

The Soviet invasion of Manchuria was the U.S.S.R.'s stepping stone to an invasion of mainland Japan. With the German front won and their Deep Battle strategy mastered, they redeployed several armies to Japan and planned a 3 front invasion of Manchuria along unexpected routes. They conquered an area the size of France, defended by 1,200,000 Japanese soldiers, in 11 days. Had the Japanese not surrendered, the U.S.S.R. would have joined the U.S. in a land invasion.